Spots, activity cycles, and differential rotation on cool stars

Alekseev, I. Yu.

doi:10.1007/s10511-005-0003-x

Cited by 11 publications

(12 citation statements)

References 16 publications

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“…The R 2 value for the least squares fit for these two periods is quite low (23.6%). It is remarkable that the obtained modulation period is very similar to values often quoted as the long cycle length of LQ Hya (e.g., Jetsu 1993;Oláh et al 2000;Berdyugina et al 2002;Alekseev 2005). However, it is known that given cycle length is nonstationary (e.g., Oláh et al 2011), partially explaining the low quality of the fit in our analysis.…”

Section: Carrier From a Multiperiodic Modelsupporting

confidence: 80%

“…These are indeed seen, but with less pronounced phase coherence. Alekseev (2005) also proposed, based on photometric spot modeling, that a latitudinal dynamo wave, the spot activity migrating from the equator poleward (the solar butterfly reversed), is present on the object. The analysis of Lehtinen et al (2012) did not reveal these trends, nor does the CF analysis picture of linear down-or upward trends support this picture.…”

Section: Discussionmentioning

confidence: 99%

“…By using local fits, e.g., Messina & Guinan (2003) give the period in the range 1. Alekseev (2005). The time span for our main homogeneous data set (D2) is around 9642 days.…”

Section: Least-squares Fittingmentioning

confidence: 99%

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Multiperiodicity, modulations, and flip-flops in variable star light curves

Olspert

Käpylä

Pelt

et al. 2015

A&A

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Aims. We study LQ Hya photometry for 1982-2014 with the carrier fit (CF) method and compare our results to earlier photometric analysis and recent Doppler imaging maps. Methods. As the rotation period of the object is not known a priori, we utilize different types of statistical methods first (least-squares fit of harmonics, phase dispersion statistics) to estimate various candidates for the carrier period for the CF method. Secondly, a global fit to the whole data set and local fits to shorter segments are computed with the period that is found to be optimal. Results. The harmonic least-squares analysis of all the available data reveals a short period, of close to 1.6 days, as a limiting value for a set of significant frequencies. We interpret this as the rotation period of the spots near the equatorial region. In addition, the distribution of the significant periods is found to be bimodal, hinting of a longer-term modulating period, which we set out to study with a two-harmonic CF model. A weak modulation signal is, indeed retrieved, with a period of roughly 6.9 yr. The phase dispersion analysis gives a clear symmetric minimum for coherence times lower than and around 100 days. We interpret this as the mean rotation pattern of the spots. Of these periods, the most significant and physically most plausible period statistically is the mean spot rotation period 1. d 60514, which is chosen to be used as the carrier period for the CF analysis. With the CF method, we seek any systematic trends in the spot distribution in the global time frame, and locally look for previously reported abrupt phase changes in rapidly rotating objects. During 2003During -2009, the global CF reveals a coherent structure rotating with a period of 1. d 6037, while during most other times the spot distribution appears somewhat random in phase. Conclusions. The evolution of the spot distribution of the object is found to be very chaotic, with no clear signs of an azimuthal dynamo wave that would persist over longer timescales, although the short-lived coherent structures occasionally observed do not rotate with the same speed as the mean spot distribution. The most likely explanation of the bimodal period distribution is attributed to the high-and low-latitude spot formation regions confirmed from Doppler imaging and Zeeman Doppler imaging.

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Section: Carrier From a Multiperiodic Modelsupporting

confidence: 80%

Section: Discussionmentioning

confidence: 99%

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Multiperiodicity, modulations, and flip-flops in variable star light curves

Olspert

Käpylä

Pelt

et al. 2015

A&A

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“…They used the contemporary Doppler images to identify the latitudes of the spots involved in producing the light modulation and the orbital migration of the phase of light minimum (φ min ) to determine the rotational period. Based on a spot modelling of the photometric data, Alekseev (2005) concluded that spots are confined to only the 30−40 • latitude region, and the active star has a solartype differential rotation; in this case also, the apparent migration of φ min in the orbital phase was used to determine the rotational period. It may be noted that φ min represents the centroid of longitudinal distribution of spots, and any change in the surface distribution will be reflected in its value, hence it is very difficult to derive the rotational period with any great certainty.…”

Section: Discussionmentioning

confidence: 99%

Orbital period modulation and spot activity in the RS CVn binary V711 Tauri

Muneer

Jayakumar

Rosario

et al. 2010

A&A

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Aims. We attempt to establish the real nature of the orbital period variation and its relation to the spot activity of V711 Tau, and determine why the (B − V) colour of the star appears to be nearly independent of its V magnitude. We wish to verify whether existing predictions in the literature for the long-term spot activity of the star are true or not by making extended photometric observations, and whether the broad component of Hα emission originates in more localised active regions, as suggested by some chromospheric models, by searching for any correlation between the base-line width of the emission and the light modulation. Methods. We obtained new radial velocities of the G-type component of V711 Tau on 42 nights during 2004−09, and BV photometry of V711 Tau on 202 nights during the years 1993−2009. We measured the equivalent widths of two well resolved lines of the G-type component from 13 of the spectra obtained by us. We also measured the equivalent widths and base-line widths of Hα emission from 21 spectra obtained during 2008−09. We analyse these along with relevant information available in the literature. Results. The available radial velocity data of V 711 Tau are consistent with a sinusoidal modulation of its orbital period; the period of modulation is found to be 36.3 ± 1.9 yr. The fractional light-loss over a photometric cycle is found to vary with an average period of 14.1 ± 0.3 yr. It appears that physical processes linked to the magnetic activity of the spotted star are responsible for the variation in the orbital period. The excess reduction in the B band flux relative to that in V band caused by spot activity is almost compensated for by the fractional increase in the contribution by the hotter companion to the total light in the blue spectral region, and thereby makes the (B − V) colour of the binary system nearly independent of its V magnitude. We find that the ratio of the radii of the components derived from their v sin i values, which is usually quoted in the literature, is substantially larger than that implied by the ratio of their brightnesses in V band. There is a slight indication from the V band data that the spot activity in V711 Tau has a component that is fixed in the orbital frame of reference. The equivalent width and base-line width of Hα emission do not show any obvious correlation with the V band light curve.

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“…Vida et al (2013) studied four systems (one K3, one M4, and the others ∼M1) with very short rotation periods (∼0.45 days) and found similarly short cycles ranging from 0.84 to 1.45 yr in all except the M4. Other nearly-fully convective stars showing signs of a cycle include AD Leo (M3; Buccino et al 2014), GJ375 (M3; Díaz et al 2007), and perhaps EV Lac (M3.5; Alekseev 2005).…”

Section: Introductionmentioning

confidence: 99%

Optical, UV, and X-ray evidence for a 7-yr stellar cycle in Proxima Centauri

Wargelin¹,

Saar²,

Pojmański³

et al. 2016

Mon. Not. R. Astron. Soc.

103

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Stars of stellar type later than about M3.5 are believed to be fully convective and therefore unable to support magnetic dynamos like the one that produces the 11-year solar cycle. Because of their intrinsic faintness, very few late M stars have undergone long-term monitoring to test this prediction, which is critical to our understanding of magnetic field generation in such stars. Magnetic activity is also of interest as the driver of UV and X ray radiation, as well as energetic particles and stellar winds, that affect the atmospheres of close-in planets that lie within habitable zones, such as the recently discovered Proxima b. We report here on several years of optical, UV, and X-ray observations of Proxima Centauri (GJ 551; dM5.5e): 15 years of ASAS photometry in the V band (1085 nights) and 3 years in the I band (196 nights), 4 years of Swift XRT and UVOT observations (more than 120 exposures), and 9 sets of X-ray observations from other X-ray missions (ASCA, XMM-Newton, and three Chandra instruments) spanning 22 years. We confirm previous reports of an 83-day rotational period and find strong evidence for a 7-year stellar cycle, along with indications of differential rotation at about the solar level. X-ray/UV intensity is anti-correlated with optical V -band brightness for both rotational and cyclical variations. From comparison with other stars observed to have X-ray cycles we deduce a simple empirical relationship between X-ray cyclic modulation and Rossby number, and we also present Swift UV grism spectra covering 2300-6000Å.

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Spots, activity cycles, and differential rotation on cool stars

Cited by 11 publications

References 16 publications

Multiperiodicity, modulations, and flip-flops in variable star light curves

Multiperiodicity, modulations, and flip-flops in variable star light curves

Orbital period modulation and spot activity in the RS CVn binary V711 Tauri

Optical, UV, and X-ray evidence for a 7-yr stellar cycle in Proxima Centauri

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